function fn(msg) {
    return new Promise((resolved, rejected) => {
        setTimeout(()=>{
            resolved(msg)
        }, 10)
    });
}

fn().then(function () {
    return fn("hello_")
}).then(value=> {
    return fn(value+"lagou_")
}).then(value=>{
    return fn(value+"I ♥ U")
}).then(value=>{
    console.log(value)
});
